1. Field of Invention
The present invention relates to a road map display device adapted for use in a navigation apparatus for an automotive vehicle, and particularly to a road map display device in which a two-dimensional coordinate of road map data is converted into a trapezoid coordinate for display of a perspective view to display a road map near the present position of the vehicle in an expanded scale and to display a distant road map in a wide range.
2. Description of Related Art
In the publication of Japanese Patent No. 2869472 issued on Mar. 10, 1999, there has been proposed a road map display device in which a portion of a road map is displayed by coordinate conversion in perspective in the form of a projective view looked at from an ostensible view point located above the portion of the road map on the earth's surface outside of a vehicle. In the road map display device, a projective conversion processing to a coordinate value of road map data is executed to convert road map data on a two-dimensional plane to data for display of a bird's eye view and to project the coordinate value of the road map data on a display screen. Accordingly, in the road map display device, calculation of a trigonometric function is required in accordance with a look down angle, and three-dimensional calculation is required in consideration with the height of the view point. For this reason, the coordinate conversion processing of the road map data becomes complicated. This results in an increase of processing load of the road map and delay of conversion processing of the data for the bird' eye view, causing difficulty in use of the display device.